Field of the Invention
The invention relates to a mixing system, which has a number of mixing elements which are disposed in a flow passage and have a number of mixing vanes disposed around a respective center axis. Adjacent mixing vanes of the mixing elements in their projection plane onto the normal plane to the center axis, in each case have an overlap. The mixing system further has at least one injection location for the injection of a reaction medium and is connected upstream of some or all of the mixing elements in a flow passage.
The mixing vane uniformly distributes the reaction medium flowing in via the injection locations. The injection location may be configured, for example, as a piece of tube with a nozzle as its terminating piece, which nozzle atomizes the reaction medium supplied so that it can be distributed more quickly and as homogenously as possible within a flow medium. By way of example, it is possible for various previously introduced gases to be mixed with one another in this way. Mixing systems of this type can also be used for the mixing of substances in liquid or dust form in a gas stream. It is also possible for them to be used in liquids. This type of mixing of substances in a flow passage is employed, for example, for a very wide range of catalysts. In particular for the catalytic reduction of nitrogen oxides contained in exhaust or flue gases using the selective catalytic reduction process (SCR process), it is necessary for a reducing agent in gas form to be added to the exhaust-gas or flue-gas stream that is to be deNOxed, specifically upstream of the catalyst. The reducing agent injected into the flue gas is usually an ammonia-containing gas, in particular ammonia-containing air or ammonia-containing recirculated flue gas. A homogenous and fine distribution of the injected reducing agent as far as possible throughout the entire flue-gas stream is important to achieve reliable catalytic conversion.
Known mixing systems contain one or two diverting elements, which are generally triangular in form and are anchored more or less obliquely in the flow passage. The diverting elements generate turbulence, which downstream leads to intensive mixing of the flow medium and all the components added.
However, complete mixing by mixing systems of this type is only achieved at a sufficient distance downstream of the mixing system or downstream of the diverting elements. This distance is generally referenced on the basis of the size of the passage cross section. In the case of gaseous substances, it is approximately 10 to 20 times the passage cross section. One drawback of this delayed mixing is that there needs to be sufficient space available downstream of the diverting elements before the subsequent components to which the mixture is to be fed can be connected. In industrial installations, however, this space is usually very tight and is not available to a sufficient extent.
To achieve faster or earlier mixing as seen in the direction of flow, there are mixing systems in which a plurality of small diverting elements are disposed next to one another perpendicular to the axis of symmetry of the flow passage. With mixing systems of this type, it is possible to achieve thorough mixing of the substances which have previously been introduced or gases which have previously been injected into the gas stream at a relatively short distance from the diverting elements. However, one drawback is that if a relatively large number of comparatively small diverting elements are used, while it is possible to compensate for local concentration differences in the substances to be mixed relatively quickly, large-volume concentration differences, for example between two opposite sides of the flow passage, can only be compensated for to an insufficient extent, since large-volume mixing does not take place within the flow passage.
Therefore, for more homogenous mixing, it is also possible to use mixing systems which, by a grid-like insert, divide the flow passage initially into a multiplicity of sub-passages. By using a corresponding inclination of the walls of the grid, in particular in the direction of flow at the end of the grid, it is possible to form these diverting elements. The diverting elements are alternately inclined in different directions, so that the flow is diverted in opposite directions by adjacent sub-passages. This leads to the substances to be mixed in adjacent sub-passages being swirled up and mixed, but the resultant swirling also causes the substances to be mixed with the flow through the remainder of the sub-passages. One drawback of a mixing system of this type is that the pressure drop caused by the mixing system is relatively high, on account of the extensive turbulence in the throughput of the individual outflow passages.